1. Field of the Invention
The invention relates generally to coated optical fibers and to a device and method for contacting and positioning lengths of uncoated optical fiber for application, by chemical vapor deposition, of amorphous or metal coatings without adverse effect upon the strength characteristics of the uncoated optical fibers. More particularly the present invention provides a coating apparatus including a plasma reactor in the form of a tubular reaction chamber having powered and grounded electrodes wound helically on the outer surface of the tubular reaction chamber. The present invention further provides a fiber guide formed by freezing a layer of liquid, preferably water, to provide a solid collar that acts as a bearing for linear movement of a fiber along a prescribed axis during application of amorphous material, including diamond-like glass coatings, inside low pressure reaction chambers.
2. Description of the Related Art
Manufacture of optical fibers, used for formation of optical fiber refractive index gratings, typically involves drawing glass filaments from highly photosensitive glass pre-forms. The process uses a down-feed system to control the rate at which the photosensitive pre-form and cladding enters the heating zone of an induction furnace. Heating zone temperatures reach from about 2200° C. to about 2250° C. Within this temperature range an optical pre-form may be drawn to the filamentary form of an optical fiber. A laser telemetric measurement system monitors the diameter of the optical fiber and its position in the draw tower. Thereafter, the newly formed optical fiber passes to one of more coating stations for application of at least one UV curable, protective coating. The protective coating, commonly referred to as a buffer coating, prevents damage that a buffer-free optical fiber may sustain by physical impact or contact with environmental contaminants including water and aqueous solutions. Historically it has not been possible to touch uncoated glass fibers without degrading their strength. For this reason, fiber optic draw towers have a height to accommodate fiber formation and application of protective buffer coatings before the optical fiber reaches the bottom of the tower. Although damage by contact with bare optical fibers can occur in a fraction of a second, buffer coatings provide sufficient protection to fragile optical fibers to allow them to be wound around storage drums and held for further processing.
Subsequent processing of a coated optical fiber may include formation of a refractive index grating in its core to produce useful articles including narrow band retro-reflectors, gain flattening devices in optical amplifiers and wavelength filters in optical communication systems. Refractive index gratings include periodic variations of refractive index that may be described as adjacent parallel planes of alternating higher and lower refractive index. The process for forming refractive index gratings requires coated optical fibers containing dopant materials, such as germanium oxide, that increase the sensitivity of the optical fiber to changes in refractive index resulting from exposure to actinic radiation e.g. from an ultraviolet laser. Without pre-sensitization, the fabrication of refractive index gratings by exposure to actinic radiation may require impractical, extended exposure times in the path of the laser beam. The degree of development and magnitude of index of refraction modulations depends upon the photosensitivity of the absorbing silica or glass structure during exposure to actinic radiation. Conventional polymeric buffer coatings, which are preferred for protecting the optical fiber, absorb actinic radiation and interfere with formation of index of refraction modulations. Removal of protective buffer, even if only from a portion of its length, returns the bare portion of the optical fiber to a vulnerable condition in which damage may occur as the stripped optical fiber undergoes modification to produce a desired refractive index grating device. Vulnerability to damage persists until the bare portion of an optical fiber receives a protective recoat of buffer material.
Formation of refractive index gratings in an optical fiber, including a glass core and an overlying cladding layer of a similar glass composition, is possible without exposing the optical fiber to impact or contaminants that might adversely affect the original physical strength characteristics of the fiber. The threat of damage may be overcome using a coating of diamond-like carbon, diamond-like glass or an amorphous coating of similar structure that forms a protective layer, transparent to ultraviolet radiation, over optical fiber cladding. Published application WO 01/66484 A1 describes diamond-like coatings and methods for their application to protect optical fibers having suitable sensitivity to radiation from an ultraviolet laser for introduction of refractive index gratings into the optical fiber core.
The challenge with the use of diamond-like coatings is the retention of the original physical strength of a drawn optical fiber during application of the coating in vapor deposition chambers operating at reduced pressure. U.S. Pat. No. 4,402,993 describes a process for coating optical fibers immediately following fiber formation using conventional drawing techniques. Optical fibers, fed directly from a fiber extruder, pass through an appropriate protective shroud to the entry of an elongated chamber having inert gas air locks at its opposite ends. Between the pressure locks, the optical fiber passes successively through a series of evacuated chamber sections. The first section comprises a plasma-ion milling zone for removing contaminants and microscopic defects from the surface of the optical fiber. Next the cleaned optical fiber passes into a second zone wherein elemental carbon, propelled in plasma-ion form, coats the surface of fiber with a diamond-like elemental carbon film of sub-micron thickness. At various points through the coating apparatus the optical fiber passes through plates having orifices of a size that is larger than the diameter of the optical fiber. To prevent damage to the fiber, by impact or abrasion against the sides of an orifice, the coating apparatus uses an inert gas positioning vortex to control gas flow between cleaning and coating zones.
The use of an inert gas vortex is one method for positioning bare optical fibers as they pass between chambers operating at different pressures. Another method uses seals that prevent gas transfer between chambers. However, during movement between pressure-controlled chambers, optical fibers roll over the seals producing a rubbing motion that could also lead to fiber weakening. A coating process typically uses monitoring equipment to sense conditions such as vibration that could damage the fiber. These precautions reduce the probability that the strength characteristics of the drawn optical fiber will decrease during application of hermetically sealed coatings.
The process described in U.S. Pat. No. 4,402,993 uses a coating chamber operating at reduced pressure to allow plasma ion formation around an electrode structure inside the coating chamber. U.S. Pat. No. 5,234,723 describes generation of plasma activated species inside a coating chamber using a single electrode wrapped spirally around the outside of the coating chamber. The use of the external electrode allows treatment of particles with plasma-activated species to apply functional coatings to the particles. Japanese published application JP 11222530 describes plasma processing to produce polymer coated metal wire passing through a tubular chamber wrapped with a single electrode. The powered electrode is twisted spirally around the outside of the tubular chamber and the metal wire provides the other electrode for plasma generation at atmospheric pressure. Japanese Published Application JP 5106053 describes generation of a low pressure glow plasma discharge following introduction of a reactive gas into one end of an insulator tube that has a pair of spirally wrapped parallel electrodes wrapped around its outer periphery. The low-pressure glow discharge plasma etches the surface of substrates inside the tube. Preferably, the insulator tube consists of glass, plastics such as PTFT, FEP, PET, PPS, PEEK, ABS, and silicone and ceramics. The spiral of the parallel pair of electrodes may be made from Cu, Ag, Ni, Al, stainless steel, carbon etc., which are separated from each other at a distance of preferably 5-20 mm. The helical wraps of the electrode pair have a separation of 20 cm. A 50 cm diameter Pyrex discharge tube 3 mm in thickness provided the insulator tube for uniform etching of silicon wafers. A low pressure glow plasma discharge, of the type described, etches substrates, such as silicon wafers, which may be stationary or moving inside or located on the inner surface of a ceramic, plastic or glass insulator tube.
A coated optical fiber having at least one protective buffer layer applied to its surface may be the starting material for a vapor deposition process that applies a coating to an optical fiber. In this case, application of a layer of diamond-like glass, for example, requires removal of any coating over the cladding of an optical fiber. The process used to remove coating from the clad optical fiber, e.g. before imprinting a Bragg grating, represents one more operation that could adversely affect the strength of a delicate single mode, optical fiber that typically comprises a core less than 1 μm in diameter and a cladding layer that increases the diameter to about 125 μm.
There is increasing use of fiber optics in applications including information transmission and optoelectronic devices. Considering the number of conditions under which damage may occur to delicate optical fibers there is a need for processes and related equipment that suppress any decrease in strength characteristics from levels associated with newly drawn optical fibers.